The present invention relates to methods for processing materials in the presence of microwave energy.
Application of microwave energy to process various kinds of materials in an efficient, economic, and effective matter, is emerging as an innovative technology. Microwave heating of materials is fundamentally different from conventional radiation-conduction-convection heating. In the microwave process, the heat is generated internally within the material instead of originating from external heating sources. Microwave heating is a sensitive function of the material being processed.
Microwaves are electromagnetic radiation with wavelengths ranging from 1 mm to 1 m in free space and frequency between approximately 300 GHz to 300 MHz, respectively. Today, microwaves at the 2.45 GHz frequency are used almost universally for industrial and scientific applications. The microwaves can be transmitted, absorbed, or reflected, depending on the material type with which they interact.
In conventional sintering processes, extremely high temperatures, long processing times and, in some cases, hot pressing or hot isostatic processing must be applied in the fabrication of products to achieve the highest density and minimum porosity. Conventional powder processing involves the compaction of a powder into the desired shape following by sintering. Typically, powders in the range of 1 to 120 micrometers are employed. The powder is placed in a mold and compacted by applying pressure to the mold. The powder compact is porous. Its density depends upon the compaction pressure and the resistance of the particles to deformation.
The powder compact is then heated to promote bonding of the powder particles. The sintering temperature is such as to cause atomic diffusion and neck formation between the powder particles. The diffusion process can yield a substantially dense body upon completion of the sintering cycle. Such a process is used in industry for a variety of products and applications.
Microwave sintering processes have unique advantages over conventional sintering processes. The fundamental difference is in the heating mechanism. In conventional heating, heat is generated by heating elements (resistive heating) and then transferred to samples via radiation, conduction and convection. In microwave heating, sample materials themselves absorb microwave power and then transform microwave energy to heat within the sample volume.
The use of microwave processing typically reduces sintering time by a factor of 10 or more. This minimizes grain growth. The fine initial microstructure can be retained without using grain growth inhibitors and hence achieve high mechanical strength. The heating rates for a typical microwave process are high and the overall cycle times are reduced by similar amounts as with the process sintering time, for example from hours/days to minutes. Microwave processing of materials has the potential to yield products having improved mechanical properties with additional benefits of short processing times and low energy usage.
Typical microwave processing procedures utilize a mixed electric field and magnetic field condition, with the sample placed into a chamber where it is exposed to both the electric field and the magnetic field generated by the microwaves. Cherrardi et al., in Electroceramics IV. Vol. II, (eds. Wasner, R., Hoffmann, S., Bonnenberg, D., and Hoffmann, C.) RWTN, Aachen, 1219-1224 (1994), published a paper indicating that both the magnetic field and electric field may contribute to the sintering of certain materials.
One embodiment relates to a process including providing a microwave radiation source and a processing chamber. The process includes generating a region of pure magnetic field from the microwave radiation in the processing chamber. A region of pure electric field from the microwave radiation is also generated. A material is positioned in the region of pure magnetic field while no portion of the material is positioned in the region of pure electric field, and the material is heated in the region of pure magnetic field. In one aspect of certain related embodiments, the heating is conducted to sinter the material. In another aspect of certain related embodiments, the material includes a metal.
Another embodiment relates to a method including providing a microwave radiation source and a processing chamber. A first region of maximum magnetic field from the microwave radiation is generated in the processing chamber, and a second region of maximum electric field from the microwave radiation is generated in the processing chamber. A body is positioned in only one of the first region and the second region during a first time period. The body is positioned in the other of the first region and the second region during a second time period. In an aspect of certain related embodiments, a first portion of the body is heated during the first time period and a second portion of the body is heated during the second time period, wherein during the first time period the first portion is heated to a higher temperature than the second portion, and during the second time period the second portion is heated to a higher temperature than the first portion.
Another embodiment relates to a method for heating including providing a substrate having a layer of material thereon. One of the substrate and the layer is positioned in one pure field selected from the group consisting of a microwave generated pure magnetic field and a microwave generated pure electric field, during a first time period. The one of the substrate and the layer is heated layer to a temperature greater than that of the other.